Optimization of two-phase flow operating parameters for making droplets in microchannel devices

Microfluidic devices are inexpensive and controllable systems as the system and the amount of fluid are contrained to the micro-scale. It is possible to use these devices in interdisciplinary fields, such as chemistry, biology, medicine, and engineering. Monodispersed droplet production in microfluidic devices has been among the most popular study topics in the recent years. Monodispersed droplets can be formed with two immiscible fluids. In general, oil (continuous phase) and water or an aqueous solution (dispersed phase) are used to form two-phase flow. It is possible to control the diameters of the droplets by changing the flow rates of the continuous and dispersed phases. The aim of this thesis study is to examine the relationship between the flow rate and the size of the droplets formed in microfluidic devices that have different geometries and dimensions by keeping the ratio of the continuous and dispersed phases constant. In the study, a simple microfluidic device with an x-junction, Type-1, and a more advanced microfluidic device, Type-2, with an auxiliary oil channel and a serpentine section integrated in the line following the x-junction were employed. Aqueous droplets in oil were produced in the devices by using oil and water flow rates with a ratio of 20:1. It was determined that when the droplet diameter covers more than half of the nozzle width, the distance between the droplets decreases as the droplet diameter increases. On the other hand, in cases where the droplet diameter is less than half of the nozzle width, it was determined that the distance between the droplets increases as the droplet diameter increases. It was determined that the droplet diameter produced in the Type-2 device, which is a more advanced device compared to the Type-1 device, is smaller than the droplet diameter produced in the Type-1 device despite the fact that both devices have the same x-junction. It was shown that the reason for obtaining different results using the same flow rates was related to the force losses and additional pressure drop encountered in the Type-2 device due to road based friction and the turns in the serpentine section. For future studies, it was desired to examine the dissolution of aqueous droplets in oil flow by heating the two-phase system with an ITO heater. The change in the diameter of the droplets was investigated by heating only the serpentine region in the microfluidic device to 40 oC. It was observed that the initial average diameter of aqueous droplets, whose solubility in oil increases as the temperature increases, decreased by 10% as they moved through the serpentine region.